The structures of cytochrome b 562 from E coli and of cytochrome b2 (L-plus-lactate dehydrogenase) from bakers yeast will be studied by X-ray diffraction methods. Both the oxidized and reduced forms of each protein will be examined. The major goal will be to understand the mechanism of electron transport at the molecular level. The structure of ferri-cytochrome minus b562 has been solved at 2.5 A resolution by single isomorphous replacement with anomalous scattering and 2-fold electron density averaging. The noncovalently bound heme ligands are histidine and methionine. The protein consists of four nearly parallel alpha-helices and resembles hemerythrin rather than cytochrome b5, c or the globins. The phases will be extended to 2.0 A and the protein refined at 1.5 A resolution. Cytochrome b2 contains both a flavin and a heme prosthetic group. It catalyzes the dehydrogenation of L-lactate, transferring electrons to cytochrome c in the mitochondria. It contains a trypsin resistant heme peptide core which resembles microsomal cytochrome b5 in sequence and spectrum. One goal of this research is to learn the relative positions of the heme and flavin groups in b2 and the structural basis for catalysis and subsequent electron transfer. A second goal is to learn the structural and evolutionary relationship bewteen b2 and b5. Cytochrome b2 crystallizes in a trigonal cell with one tetramer of 230,000 Daltons per asymmetric unit. The isomorphous replacement method of structure investigation will be coupled with the molecular replacement method to solve this structure. The structure-function relationships and comparison with other proteins, as well as electron density fitting, will be carried out on an MMS-X molecular graphics system.